Make the time spend waiting for things to settle down configurable

[JensGutermuth]

I had some trouble while requesting my first wildcard certificate, because Let's Encrypt checked the challenge to quickly. I thought making it the time spend waiting for things to settle down configurable was the most sensible option to avoid these problems in the future. I currently use 30s.

[captn3m0]

I like this change as I've faced similar issues with the hook.

[YasharF]

This repo is no longer actively maintained. https://github.com/SeattleDevs/letsencrypt-cloudflare-hook is the maintained forked. I have cherry picked your commit and added it to that repo. Feel free to open future PRs in that repo.